I have been using it to do some product shoot tests (models built in Alias Studio, exported to Proe for final touches, re-exported to Maya where I set up the Maxwell scene).
The results are pretty good if you are willing to put in the rendering time (12-16h for my tests at 2000x1000). The setup is fairly easy and straight forward compared to other rendering applications.
I don't mind long rendering times if the setup only takes a few minutes - but maybe not 12-16h. It's still in beta so am sure the final release will see someperformance improvements.
Since Maxwell is based on real physics, cheats to produce some effects or looksare not available. Just something to keep in mind depending the results one might want to achieve.
Supposibly Maxwell will be a standalone application but it has not yet been shown. Currrently a few plug-ins are available (Maya, Cinema4D, Max, Rhino, SolidWorks, etc. - see website for complete list).
The current User Guide is available from the Product page. You can take a look at the interface (for the plug-ins that is) there.
Next Limit will probably have a demo version of Maxwell available when they release version 1.0 of Maxwell. The current due date is in October I believe.